Cam for internal combustion engines



nited States Patent CAM FOR INTERNAL coMBUsrIoN ENGINES Wolf-DieterBensinger and Dietrich Kurz, Stuttgart-Unitartnrirhei Germany, assignorsto Daimler-Benz Alttiengeseiischaft, Stuttgart-Unterturkheim, GermanyApplication September 22, 1953, Serial No. 381,602 Claims priority,application Germany September 22, 1952 4 Claims. (Cl. 123-90) Thepresent invention relates to a cam for an internal combustion engine,such as used for actuating a springpressed poppet valve or for actuatingspring-controlled valve pistons, e. g. the pistons of fuel injectionpumps or feed pumps.

The actuation of a spring-pressed reciprocatory element, such as apoppet valve, comprises two phases: the acceleration and thedeceleration of the member. During the deceleration phase the springmust positively maintain engagement of the reciprocatory member with thecam. For that purpose, the variable force produced a permitting thespring to be rendered as light as possible.

More particularly, it is an object of the invention to reduce themaximum force exerted by the spring maintaining engagement between camand actuated member to a minimum as is desirable in order to reduce theweight, cost and size of the spring and in order to obtain favorableconditions with respect to natural oscillations of the spring.

According to the present invention, the cam has a profile shaped toproduce a movement of the actuated memher which, when represented in arectangular coordinate system as a function of the angular position ofsaid cam, is indicated by a curve which, within the deceleration phase,is composed of oblique sine curves and of a polynomialcurve of a highdegree. As a result, a cam i s ob tained with which the actuated member,such as the poppet valve, can be kept in continuous engagement at a highrotary speed of the cam with a comparatively light spring. Moreover, thecam may be readily so designed as to comply with the specificrequirements of any particular case.

The present invention will be described hereinafter in detail withreference to a specific embodiment thereof, it being understood,however, that such detailed description serves the purpose ofillustration rather than that of limitation of the invention. In thedrawing Fig. 1 illustrates a section through a cam shaft and aspring-pressed poppet valve of an internal combustion en ine actuatedthereby, the invention being applicable to the cam of such cam shaft,

Fig. 2 shows a detail of Fig. 1 on an enlarged scale illustrating thecam shaft, 7

Fig. 3 is a graph showing the lift of the valve represented in arectangular coordinate system as a function of the angular position ofthe cam shown in Fig. 2, and

Pig. 4 is a graph showing the deceleration of the valve represented in arectangular coordinate system as a function of the angular position ofthe cam.

The poppet valve P which may be the inlet valve or the outlet valve ofan internal combustion engine, is subjected to the force of a spring Swhich rests on a shoulder of the cylinder block and acts on a springplate fixed to the valve stem thereby tending to keep the valve closed.Contrary to the force exerted by the spring S, the valve P may bepositively opened by action of a cam C forming part of a rotating camshaft journalled in the cylinder block, such cam C acting upon a plunger1 interposed between the cam C and the poppet valve P. The spring S mustbe so dimensioned as to maintain uninterrupted engagement between thestem of valve P, plunger I and cam C.

The object of the present invention is to so design the cam C that at acertain rotary speed of the cam shaft such engagement will be maintainedwith a minimum of spring power.

0, l, 2, 3 and 4 are critical points on the periphery of the lobe of thecam which will act on the plunger I successively. During the first phaseof the cam actuation, while the portion of the periphery between thepoints 0 and 1 acts on the plunger, the actuated member I, P will beaccelerated up to its highest speed which is reached when the point 1 ofthe cam periphery contacts the plunger. In the following phase whichwill be called the deceleration phase hereinafter, the speed of theactuated two-part member I, P is lowered to a full stop and is thenPatented Sept. 3, 1957 reversed until the downward speed of the closingmove-- ment of the valve attains a maximum. This happens when point 4 ofthe cam contacts plunger I.

The valve lift L produced by the cam is represented in Fig. 3 in arectangular coordinate system as a function of the angular position T ofthe cam. Since the cam revolves at a constant speed, the abscissaerepresenting the angular position of the cam will also represent thetime. In Fig. 3 the deceleration period extends from instant 1 toinstant 4. According to the present invention, the lift curve in Fig. 3within the deceleration phase between 1 and 4 is composed of obliquesine curves 5 and 6 joined I at the points 7 to a polynomial curve 8.

Preferably, the polynomial curve is of the fourth de is y=fsin gx, inthis equation h, f and g being constants.

The constants a to h are so chosen that the joined ends o f the curves5, 6 and 8 at 7 have identical inclination and curvature. Preferably,each of -the oblique-sine curves 5 and 6 has a length of one quarter ofa period.

The deceleration force which must be exerted upon the reciprocatorymember I, P in order to impart the movement thereto illustrated by thediagram in Fig. 3 is represented by the curve 9 in Fig. 4. As is wellunderstood in the art, the deceleration curve 9 represents the secondderivative of the lift curve 5, 8, 6. Hence, a force as represented bycurve 9 would be sufiicient if exerted upon the reciprocatory member I,P to maintain the same in permanent engagement with the cam when thesame revolves at the permissible maximum speed. In fact, however, thespring S exerts a higher force which varies as the spring is compressedand relaxed. Such force would be capable to impart a higher decelerationto the reciprocatory member I, P, such higher deceleration beingindicated by curve 10 in Fig. 4. Since the force is proportional to theproduct of the mass and the acceleration or deceleration, curve 10represents the actual force produced by spring S divided by the mass ofthe member I, P. Expressed in other words, curve 10 represents themaximum acceleration which spring S could impart to the poppet valvemember I, P in the direction towards the cam theoretically in view ofthe mass of such member.

This acceleration has a maximum value when the spring I is compressedor, in other words, when the tip of the cam lobe acts on plunger 1.

It is another characteristic of the present invention that the'twocurt/cs9 and 10 are nearly equidistant over most of the decelerationphase. The distance of the curves 9 and 10 denotes a surplus of forcewhich will guarantee the permanent engagement between cam and plunger.

The equidistant relationship of the two curves offers the advantage thatwith a given acceleration'from 0 to 1 and with a given total lift androtary speed of the cam, a minimum of spring force is sufficient tomaintain permanent engagement. An analysis of dilferent possibilities ofreducing the lifting speed reached at point 1 in Fig. 2 down to zeroreached at the center line of the diagram shows that the areas confinedbetween the elected deceleration curve and the abscissa is invariablythe same in all cases. In Fig. 4 the curves 11 and 12 have been drawn asrepresenting theoretical deceleration-s confining the same areas ascurve 9 and approaching extreme conditions. It will appear that ineither case curve 10 representative of the size of the spring would haveto be moved down in order to secure the same surplus 13 of spring power.Therefore, a deceleration curve, such as 11, or a deceleration curve,such as 12, or any deceleration curve therebetween except curve 9 wouldrequire a more powerful spring.

While we have described our invention with reference to a preferredembodiment thereof, we wish it to be clearly understood that the same isin no way limited to the details of such embodiment, but is capable ofnumerous modifications within the scope of the appended claims.

What we claim is:

- 1. A mechanism of an internal combustion engine including a curvedcam, a reciprocatory member and a spring normally urging said memberinto engagement with said cam, said cam having a profile shaped toproduce a reciprocatory movement of said member and said springsurpassing by a certain safety amount the force required to deceleratesaid member in accordance with the shape of said cam thereby positivelymaintaining in engagement said member with said cam; said reciprocatorymovement, when represented in a rectangular coordinate system as afunction of the angular position of said cam, is indicated by a curvewhich, within the deceleration phase, is composed of oblique sine curvesjoined to a polynomial curve of high degree, the joined ends of saidcurves having identical inclination and curvature.

2. A mechanism oftan internal combustion engine including a curved cam,a poppet valve and a spring normally urging said valve into engagement:with said cam, said cam having a profile shaped to produce a lift ofsaid valve and said spring surpassing by a certain safety amount theforce required to decelerate said valve in accordance with the shape ofsaid cam thereby positively maintaining in engagement said valve withsaid cam; said reciprocatory movement, when represented in a rectangularcoordinate system as a function of the angular position of said cam, isindicated by a lift curve which, within the deceleration phase, iscomposed of oblique sine curves joined to a polynomial curve of highdegree, the joined ends of said curves having identical inclination andcurvature.

3. A mechanism of an internal combustion engine including a curved cam,a poppet valve and a spring normally urging said valve into engagementwith said cam, said earn having a profile shaped to produce a lift ofsaid valve and said spring surpassing by a certain safety amount theforce required to decelerate said valve in accordance with the shape ofsaid cam thereby positively maintaining in engagement said valve withsaid cam; said reciprocatory movement, when represented in a rectangularcoordinate system as a function of the angular position of said cam, isindicated by a lift curve which, within the deceleration phase, iscomposed of oblique sine curves joined to a polynomial curve of thefourth degree constituting the crest of said lift curve, the joined endsof said curves having identical inclination and curvature and each ofsaid oblique sine curves has a length of one quarter of a period.

4. A mechanism of an internalcombustion engine including a curved cam, apoppet valve and a spring normally urging said valve into engagementwith said cam, said cam having a profile shaped to produce a lift ofsaid valve and said spring surpassing by a certain safety amount theforce required to decelerate said valve in accordance with the shape ofsaid cam thereby positively maintaining in engagement said valve withsaid cam; said reciprocatory movement, when represented in a rectangularcoordinate system as a function of the angular position of said cam, isindicated by a lift curve which, within the deceleration phase, iscomposed of oblique sine curves joined to' a polynomial curve of thefourth degree constituting the crest of. said lift curve, the joinedends of said curves having identical inclination and curvature and. eachof said oblique sine curves has a length of one quarter of a period andwithin the deceleration phase the second derivative. of said lift curveextends substantially parallel to a curve representing the force of thespring divided by the mass of said poppet valve.

References Cited in the file of this patent UNITED STATES PATENTS2,628,605 Jones Feb. 17, 1953

